1. Field of the Invention
The present invention relates to non-asbestos friction materials highly suitable as disc pads, brake linings, clutch facings and the like in automobiles, heavy duty trucks, railroad cars, and various types of industrial equipment.
2. Prior Art
Friction materials used for braking in automobiles, heavy duty trucks, railroad cars, and various industrial equipment are desired to have a number of performance features. These features include excellent wear resistance, a friction coefficient that is both high and stable, outstanding resistance to brake fade, no generation of undesirable noises such as squeal during brake operation, resistance to wear debris formation, and short-time moldability.
For example, several attempts have been made to find a way to reduce brake squeal. These include the addition of rubber powder and the use as a binder of rubber-modified phenolic resins such as acrylic rubber-modified phenolic novolac resins, which have a low heat history-associated decline in damping properties, or nitrile rubber-modified phenolic novolac resins, which are endowed with excellent damping properties at ambient temperatures. The aim in this prior art has been to impart damping properties in order to increase vibration damping and reduce squeal (see, for example, JP-A 60-184533 and JP-B 60-51504).
However, nitrile rubber-modified novolac phenolic resins undergo a decline in vibration damping performance on account of heat history-associated changes over time, resulting in squeal. In addition, they have inadequate heat resistance. Adding this type of resin in an amount sufficient for noise suppression leads to a decline in braking performance. Acrylic rubber-modified novolac phenolic resins do have an excellent heat resistance, but have a poor moldability. Specifically, when these resins are molded under applied heat and pressure, blisters form and internal cracking tends to arise.
In order to enhance the manufacturability of friction materials, use has recently been made of high-ortho-novolac phenolic resins, which can be molded in a short period of time. Unfortunately, while the use of high ortho-novolac phenolic resins as the binder in friction materials does improve the rapid curability of the friction material composition, the molded article becomes harder and thus more likely to generate unwanted noise during brake operation.
All prior-art friction materials thus have various drawbacks which prevent them from fully satisfying the desire for better performance.
Therefore, one object of the present invention is to provide a non-asbestos friction material that has excellent rapid curability, enabling the molding cycle time to be shortened, and that essentially does not squeal during brake operation.
Another object of the invention is to provide a high-quality non-asbestos friction material endowed with robust vibration damping characteristics that are resistant to heat history-associated deterioration over time, minimal brake noise, and the ability to maintain these performance features over an extended period of time.
The inventor has learned that a noise suppressing effect is achieved when the value for the 100 Hz vibration damping factor (tan xcex4) at 300xc2x0 C. minus tan xcex4 at 50xc2x0 C. is at least xe2x88x920.030. That is, because it is believed that a higher vibration damping factor is associated with less squeal and because ordinary friction materials show a marked decline in the vibration damping factor with rising temperature, the vibration damping factor at 50xc2x0 C. has customarily been set at a relatively high value. Thus, a comparison of tan xcex4 at 50xc2x0 C. (V50) with tan xcex4 at 300xc2x0 C. (V300) in prior-art friction materials would show a large difference between V50 and V300. Specifically, V300 is much smaller than V50, with the difference V300xe2x88x92V50 often being more negative than xe2x88x920.030. On account of this, the practice in the art has been to set V50 at a high enough value to ensure that V300 can still be maintained at or above a given level even if it decreases relative to V50.
However, the inventor has found that when the difference V300xe2x88x92V50 is at least xe2x88x920.030 (i.e., when the degree of decline of a vibration damping factor is so low that the difference of V300 from V50 is small or when V300 is larger than V50 (the difference V300xe2x88x92V50 is positive)), surprisingly, squeal is markedly suppressed even if V50 is set at about the same value as the prior-art V300 value rather than being set at a higher value according to the conventional practice.
Although it is not fully understood why there is such a noise suppressing effect when the decrease in the vibration damping factor is small or the damping factor becomes larger, the reasons generally appear to be as follows.
(a) During use, the temperature of the friction material itself varies due to friction. It is conceivable that when a material which does not squeal at a given temperature is selected, it will squeal at some other specific temperature. Given that squeal appears to be associated in some way with vibration damping, materials that undergo only small changes in the vibration damping factor relative to temperature changes are probably effective against squeal.
(b) Assuming squeal to be a form of resonance, a high vibration damping factor in combination with, as noted above, minimal change in the vibration damping factor due to changes in conditions should be effective for maintaining the initial performance, and ultimately for eliminating squeal.
The inventor has also found that non-asbestos friction materials made by molding and curing a composition comprised of a fibrous base, a binder and a filler, in which the binder is (1) a rubber-modified high-ortho phenolic resin, (2) a resin mixture of a rubber-modified high-ortho phenolic resin and a rubber-modified phenolic resin, or (3) a resin mixture of two or more rubber-modified phenolic resins, and especially (1) an acrylonitrile-butadiene rubber-modified high-ortho phenolic resin alone, (2) a resin mixture of an acrylonitrile-butadiene rubber-modified high-ortho phenolic resin and an acrylic rubber-modified phenolic resin, or (3) a resin mixture of an acrylic rubber-modified phenolic resin and a nitrile rubber-modified phenolic resin, exhibit little change in the vibration damping factor (tan xcex4) and are effective for preventing squeal during brake operation. Moreover, these friction materials provide all desired performance features including wear resistance, functional stability, moldability, and fade resistance. These asbestos-free friction materials are able to effectively resolve the outstanding problems in the prior art.
Thus, according to a first aspect of the invention, there is provided a non-asbestos friction material made by molding and curing a composition comprising a fibrous base, a binder and a filler, the friction material having a 100 Hz vibration damping factor (tan xcex4) at 300xc2x0 C. minus tan xcex4 at 50xc2x0 C. value of at least xe2x88x920.030
According to a second aspect of the invention, there is provided a non-asbestos friction material made by molding and curing a composition comprising a fibrous base, a binder and a filler, wherein the binder is a rubber-modified high-ortho phenolic resin.
According to a third aspect of the invention, there is provided a non-asbestos friction material made by molding and curing a composition comprising a fibrous base, a binder and a filler, wherein the binder is a resin mixture of a rubber-modified high-ortho phenolic resin and a rubber-modified phenolic resin.
According to a fourth aspect of the invention, there is provided a non-asbestos friction material made by molding and curing a composition comprising a fibrous base, a binder and a filler, wherein the binder is a resin mixture of an acrylic rubber-modified phenolic resin and a nitrile rubber-modified phenolic resin.